What about and even more scaled down version that uses the existing Comfort IO's and cannot expand beyond that? All that you then need is a KA/KB and 12V connections and if that fits inside a DIN rail mount it would be awesome. See Clipsal's PAC module as a reference for size.

The logic engine currently uses the left part of the Comfort PCB without any changes except an external power supply is used and the firmware is different
If there is demand for a product in such a form factor a DIN rail product may be developed in future